Highly sensitive NO2 sensors using lamellar-structured WO3 particles prepared by an acidification method

Tungsten trioxide (WO3) was prepared by acidification of Na2WO4 with acid solutions such as H2SO4, HCl, and HNO3 (pH 0.5 to −0.8) and tested for its NO2 sensing properties. Acidification with strong acid solutions (pH −0.5, −0.8) was found to produce lamellar-structured WO3 particles, which consisted of nano-sized crystalline plates that were 100–350 nm in lateral size and 20–50 nm in thickness, as observed by XRD and SEM analyses. The sizes of the primary and secondary particles were decreased by decreasing the pH of the acid solution used. This was accompanied by an increase in the specific surface area. The NO2 responses of the prepared WO3 lamellae were dependent on their morphology. The device using smaller WO3 lamellae prepared with a H2SO4 solution (pH −0.8) had the highest sensor response, exhibiting a high sensor response (S = 150–280), even to dilute NO2 (50–1000 ppb) in air at 200 °C. The use of smaller lamellae resulted in a decrease in the electrical resistance of the device, probably due to intimate contact between smaller lamellar particles, which allowed the detection of NO2 in a rather wide concentration range. In addition, the developed device showed high NO2 selectivity without substantial interference from NO.